The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which form the anode and cathode electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit connected therebetween. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or separator plates to complete a single PEM fuel cell. Separator plates serve as current collectors for the anode and cathode, and have appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes.
In practice, however, PEM fuel cells are not individually operated. Rather, PEM fuel cells are connected in series, or stacked one on top of the other, to form what is usually referred to as a fuel cell stack. PEM fuel cell stacks are generally loaded in compression in order to maintain low interfacial electrical contact resistance between the separator plates, the gas diffusion media, and the catalyst electrode. The low interfacial contact resistance in a PEM fuel cell stack is directly related to the compression loading. Typically, compression loads on the separator plate range from about 50 to about 400 psi and are controlled by a compression retention system. Importantly, such systems are often installed under an even higher building load to compensate for loss in compression that occurs when the building load is removed.
Compression retention systems are typically designed in a manner effective to offset strains produced by membrane swelling and compressive stress relaxation in the fuel cell stack. Such systems act to minimize an over-compression and damage of gas diffusion media in the fuel cell stack, as well as maintain the stack compression and contact pressure between separator plates, gas diffusion media, and catalyst layers. It is disclosed by Gibb et al. in U.S. Pat. No. 5,484,666 that conventional compression systems have consisted of tie rods extending through and between end plate assemblies secured with fastening nuts. Springs threaded on the tie rods and interposed between the fastening nuts and the end plates have been used to apply resilient compressive force to fuel cell stacks in the stacking direction.
In addition to compression retention systems, conventional PEM fuel cell assemblies include delivery subsystems for distribution of hydrogen fuel, oxidant and coolant to the fuel cell stack. For example, devices such as manifolds with ports for directing gases and fluids to the interior of the stack are common. Subsystems for exhausting reaction products and coolant are also generally present. Further found within fuel cell systems are current collectors, cell-to-cell seals, insulation, pumps, fans, valves, compressors, associated plumbing, electrical connections and instrumentation. Such subsystems and devices are volume consuming, can represent increased thermal mass when located outside the stack (requiring more time to warm the stack to appropriate temperatures), and in some cases can cause an electrically parasitic load on the fuel cell stack.
Peripheral preconditioning devices have also been necessary for optimum operation and performance of a fuel cell stack. Such systems can include, for example, reformers for extracting usable hydrogen fuel from hydrogen-containing feedstock. Additionally, humidifiers for wetting the PEM layers of the fuel cell stack and facilitating conduction of protons from the anode layers to the cathode layers of the MEA are often necessary. These peripheral devices require extensive additional hardware which can also lead to poor system efficiency. This poses problems in many applications, such as vehicular applications, where it is desirable that weight and size of a fuel cell system be minimized.
There is a continuing need for a fuel cell system that is volumetrically and thermally efficient, less massive, and integrates many of the aforementioned fuel cell subsystems, including compression retention systems. A method for manufacturing such fuel cell systems that minimizes loss in compressive force following a build is also desired.